It is known in the art that a stress corrosion crack (SCC) occurs in a welded portion of a recirculating pipe of a boiling water type light-water reactor plant. On the other hand, already-existing thermal power generating facilities are toward age deterioration; in actual fact, Western countries have experienced accidents in which high-temperature steam pipes of aging thermal power plants ruptured due to cracking in a heat affected zone caused by the occurrence and joining together of creep voids.
An ultrasonic test has been conducted for nondestructive examination of a thick welded portion of a pipe as in a nuclear power plant. For the nondestructive inspection of the thick welded portion of the pipe, there is a growing demand for accurate determination of flaw height as well as for flaw detection. And flaw sizing requires the detection of start and stop points of the flaw. In, recent years, the need for high-accuracy measurement of flaw height and early detection of the flaw has become so intensified that the application of a phased array method or a TOFD (Time of Flight Diffraction) method is now under consideration.
The nondestructive examination by ultrasonic test usually employs a flaw height measuring method utilizing tip echoes. In general, tip echo techniques have been widely used so far. FIG. 15 illustrates a typical example of the tip echo technique that employs an angle beam probe (an angle beam method). In the conventional flaw height measuring method using tip echoes, an angle beam probe 101 is moved little by little to determine the position of maximum intensity of a reflected wave 104 from an open end 103 of a flaw 102 (which reflected wave is commonly called a corner echo) (see FIG. 16(A)), followed by, determining the position of maximum intensity of a reflected wave 106 from the top end 105 of the flaw 102 (which reflected wave is commonly called a tip echo; and the top end will hereinafter be referred to simply as a tip, except where specifically noted) (see FIG. 16(B)). The flaw height h is computed from the difference between arrival times t1 and t2 of the two echoes 104 and 106. In this instance, when the center axis of the ultrasonic beam coincides with the top end 105 and the open end 103 of the flaw 102, the heights of the echoes 104 and 106 corresponding thereto are maximum. In FIG. 15, reference numeral 107 denotes, the transmitted ultrasonic beam, and 101′ and 107′ denote the angle beam probe and the ultrasonic beam, respectively, at the time of coincidence between the center axis of the ultrasonic beam and the top end 105 of the flaw 102.
Accordingly, the two echoes 104 and 106 will not be received simultaneously with the same intensity. FIG. 8 shows the results of simulation of waveforms received by the probe that was shifted a distance L at one time toward the slit from the position where the reception of the corner echo was began. As is evident from FIG. 8, when the probe 101 is moved toward the slit 102, the amplitude of the corner echo 104 becomes maximum first, and when the probe 101 is brought closer to the slit 102, the amplitude of the tip echo 106 increases. Letting rise times of the tip and corner echoes 106 and 104 of the maximum amplitudes be represented by tt and tc, respectively, beam paths for the both echoes are given by the following Equation 1:Wi=Cti/2, where i=t, c.  <Equation 1>In the above, C is the velocity of sound, which is the velocity of a shear wave in the above example.
Then, beam paths Wt and Wc of the tip and corner echoes 106 and 104 are calculated, and from their geometrical relation the flaw height h can be obtained by the following Equation 2:h=(Wc−Wt)cos θ  <Equation 2>where θ is the angle of refraction. FIG. 7 shows, by way of example, predictions, by an ultrasonic wave FEM (Finite Element Method) simulation, about ultrasonic wave fronts in the case where the slit tip happens to be on the center axis of an ultrasonic beam launched by a shear 45° angle beam probe (Position 1 in FIG. 15). As seen from FIG. 7, after the shear wave reached the slit tip, diffracted longitudinal and shear waves spread out in circular arc form from the slit tip and the diffracted wave returning to the probe is received as the tip echo.
It is the TOFD method that measures the flaw height by disposing a transmitting probe 201 and a receiving probe 202 such that the latter receives a diffracted wave 206 which propagates opposite to a transmitted wave 207 when viewed from the flaw tip 105 as depicted in FIG. 17. With the TOFD method, the flaw height h can be calculated, by the following Equation 3, from the beam path of the diffracted wave 206.h=T−Wt sin(cos−1(Ws/2Wt))  <Equation 3>where T is the thickness of a specimen and Ws is the beam path of a surface wave.    Non-patent document 1: Corp. Japanese Society for Nondestructive Testing, “Flaw Height Measuring Method by Tip Echo Techniques Standardized by Japanese Society for Nondestructive Testing”    Non-patent document 1: Corp. Japanese Society for Nondestructive Testing, “Flaw Height Measuring Method by TOFD Method Standardized by Japanese Society for Nondestructive Testing”